Miniaturized time-lag cartridge fuse having ribbon-type fuse link



y Jan. 28, 1969 RIBBON-TYPE E LINK Filed Sept. 1967 lll/11171114@ 4alllm|ln (Ill/11111,'lll/11411111 F. J. KozAcKA 3,425,018 MINIATURIZED TIME-LAG CARTRIDGE FUSE HAVING INVENTOR:

FREDERI BY MMM CK J. KOZACKA United States Patent Office 3,425,018 Patented Jan. 28, 1969 6 Claims ABSTRACT F THE DISCLOSURE A static miniature time-lag fuse wherein circuits are interrupted on occurrence of protracted overloads by a metallurgical reaction between the base metal of the fuse link and a low melting point overlay metal rather than by relative movement or separation of a pair of cooperating contacts normally conductively interconnected by a joint of soft low fusing point solder.

The term minature fuses is used in this context with reference to fuses having smaller sizes than National Electrical Code fuses having7 the same current rating and the same voltage rating.

Background of invention The Military Standard Style F09 fuse is a family of fuses complying with Military Specification MIL-F- 15160 D, Supplement 1, Feb. 1, 1961. While fuses of this description originated with the military, they are also widely used in connection with non-military applcatons on account of their compactness.

The severe physical and electrical requirements imposed upon Military Standard Style F09 fuses-hereinafter brieliy referred-to as Style F09 fuses-made it heretofore impossible to evolve static Style F09 fuses which were entirely satisfactory. As a matter of fact, it was not even known heretofore whether or not satisfactory static Style F09 fuses could be evolved, i.e. static Style F09 fuses meeting all requirements imposed upon such fuses. I have discovered that this can be done, and this discovery is based on the discovery of critical ranges of parameters.

Style F09 fuses must have a length of 11/2 |1/,2 inches. The casing must have an outer diameter not exceeding 0.4 inch, but not less than .370 inch, and is generally of fiber. The ferrules mounted on the casing for closing the ends thereof must overlap the side wall of the casing a distance not exceeding 0.406 inch, and their diameter must be 04061-0004 inch. The fuses must have a predetermined time-current ourve involving a significant time-lag in the overload blowing range. To be more specic, at 135 percent nominal current rating the blowing time must be 0-1 hour, at 200 percent nominal current rating the blowing time must be l2 seconds, and at 300 percent nominal current rating the blowing time must be 6 seconds. In addition to overload currents Style F09 fuses must -be capable of interrupting major fault currents, i.e. in D.C. circuits having a circuit voltage of 250 volts they must be capable of clearing fault currents up to 10,000 amps. The same physical and electrical characteristics must be applicable to a wide range of nominal current ratings, c g. amps to 30 amps.

These limitations coupled with these requirements are not paralelled in any prior art static time-lag fuse, The development of static time-lag fuses is normally based on empirical data which are modified, or extrapolated, to comply with the requirements of any specific new development, But static Style F09 time-lag fuses are so radically ditferent from prior art static time-lag fuses that there are no empirical data on which the development of this type of fuses could be based.

Neither a conventional wire type fusible element with a wire-severing low melting point metal overlay, nor a conventional ribbon type fusible element with a ribbonsevering low fusing point metal overlay, will establish the required performance characteristics, whatever dimensions may be given to either type of fusible element. There are empirical data lavailable relative to requirements of serially related points of break depending upon circuit voltage, nature of the arc-quenching filler, etc. I have found that these data are not applicable, or do not apply, to static Style F09 time-lag fuses, mainly because of their compactness, and the resulting increase of ion concentration incident to blowing.

The burnback of a fusable element depends inter alia upon the current density therein, the nature of the pulverulent arc-quenching filler in which the fusible element is submersed, and the metal of which the fusible element is made. In order to develop a novel fuse structure cmpirical data in regard to burnback is needed. I have found that the fusible element of a Style F09 time-lag fuse cannot be made of any metal normally used for making fusible elements, and for which burnback conditions have been explored. If the fusible element for a static Style F09 time-lag fuse is made of a metal conventionally used for making fusible elements, e.g. silver, copper, or zinc, the fusible element tends to become too fragile for handling and its mass too small to achieve the required time-lags. If, on the other hand, the fusible element were made of a high resistance metal mainly intended for making resistors, it is not possible to rapidly sever such metals by a metallurgical reaction initiated -by an overlay of a metal'having a lower lmelting point than the -base metal of the fusi-ble element, i.e. fusible elements made of such rnetals cannot be severed suiiieiently rapidly by a metallurgical reaction following a predetermined time delay, or time-lag. In order to achieve long time-delays the heat storage capacity of the fusible element must be relatively large, and cooling by heat flow away from the fusible element must be relatively small, yet these requirements are strongly counteracted by the small size of Style F09 fuses.

All this raises two fundamental questions, namely (a) is it possible to evolve static Style F09 time-lag fuses?, and (b) what materials are required for such fuses, and what dimensions must be given to the constituent parts thereof, assuming the choice of materials and that of dimensions to be critical? Summary of invention This invention consists in discovering that it is possible to evolve satisfactory static Style F09 fuses depending on the use of critical materials, and maintenance of critical dimensions.

The fuse link conductively interconnecting the ferrules must 'be of the ribbon type and of phosphor bronze having a small electrical conductivity, e.g. 11% IACS at 68 deg. F. and have substantially at the center thereof a linksevering overlay including tin. Links of the ribbon type made of small conductivity Iphosphor bronze are mechanically strong enough to be safely handled even if designed for small current ratings, eg. 5 amps, and can relatively quickly be severed by -metal interdiffusion processes.

The fuse link must have a width of about 0.150 inches. This critical width depends upon other critical parameters, and it makes it possible to vary the nominal rated current of the fuse between the limits of 5 amps and 30 amps without any change other than varying the thickness of the bronze sheet out of which the ribbon fuse link is stamped.

The casing of the fuse must be filled with, and the ribbon fuse link submersed in, a pulverulent arc-quenching liller having a substantially lower thermal conductivity than the thermal conductivity of tine quartz sand, and

3 about the same thermal conductivity as powdered gypsum having a particle size of 18-20 U.S. Standard Sieve Number. The pulvemlent arc-quenching filler is preferably, though not necessarily, powdered gypsum having a particle size of 18-20 U.S. Standard Sieve Number.

The fuse link must define four serially 4related points of reduced cross-sectional area each having a width of about 0.04 inches. This number of serially related points of reduced cross-sectional area is in excess of the number of points `of reduced cross-sectional normally provided in ribbon fuse links intended for fuses having the rated voltage of Style F09 fuses, i.e. 250 volt.

The spacing of each of the axially outer of said four points of reduced cross-sectional area from the end surface of the ferrule immediately adjacent thereto must be about 0.23 inch` This distance is critical to the extent that a change of, or exceeding, i/gg inch will significantly alter the performance of the fuse.

The mass of the ribbon fuse link and the mass of the link-severing overlay must be sufiiciently large to preclude interruption of the current path formed by the fuse link as a result of melting of the link-severing overlay in times shorter than 6 seconds at currents 3 times the rated current of the fuse. In this context the rated current of a fuse means a current which the fuse is capable of carrying continually, or for an unlimited period of time, without exceeding ambient temperature by 50 deg. C., or more than 50 deg. C.

`Brief description f drawings FIG. l is a section along I-I of FIG. 2 and shows a fuse which is a stage in the development of the fuse embodying this invention;

FIG. 2 is a section along II-II of FIG. 1;

FIG. 3 is a section along III-III of FIG. 4 and shows a fuse embodying this invention;

FIG. 4 is a section along IV-IV of FIG. 3;

FIG. 5 is a top-plan view of the fuse link of the fuse of FIGS. 3 and 4; and

FIG. 6 is a side elevation of the fuse link structure of FIG. 5.

Description of preferred embodiment of invention As mentioned above, FIGS. l and 2 illustrate a fuse which does not embody this invention, but is a stage in the development of fuses according to this invention. A description of the stage shown in FIIGS. l and 2 is conducive to a better understanding of the invention itself.

The fuses shown in all figures are Standard Style F09 fuses shown on a larger scale than the standard size. Reference numeral 1 has been applied to indicate la tubular casing of Ian electric insulating material having `a length of 1-1/2 ilg inches and an outer diameter not exceeding 0.400 inch, and being not less than 0.370 inch. A pair of ferrules 2 is mounted on casing 1 and closes the ends thereof. Each of ferrules 2 overlaps the side wall of casing 1 a distance not exceeding 0.406 inch. Casing 1 is lled with a pulverulent arc-quenching filler 3 having a relatively small thermal conductivity, i.e. a thermal conductivity of the same order as powedered gypsum having a particle size of 18 to 20 U.S. Standard Sieve Number. A ribbon link 4 of a phosphor bronze having a small electrical conductivity conductively interconnects ferrules 2. Ribbon fuse link 4 has a width of about 0.150 inch. Its ends project through slots in ferrules 2 into pools 5 of solidified soft solder received in recesses of ferrules 2. Fuse link 4 has two circular rperforations 4a having a diameter of 0.124 inch. A link-severing overlay 6 of tin is arranged in the center of link 4 between perforations 4a thereof. Overlay 4a extends to the points of minimal cross-section of link 4, but not beyond these points. The reasons underlying this configuration of overlay 5 are more fully set forth in U.S. Patent 2,988,620 to Frederick I. Kozacka, issued lune 13, 1961, for Time-lag Fuse, and reference may be had to that patent for further details regarding the configuration of overlay 6.

The fuse structure of FIGS. 1 and 2 is the outgrowth of an extended investigation whose object it was to develop a static Style F09 fuse. The fuse of FIGS. 1 and 2 which has been described above is capable of meeting the tme current curve requirements set for Style F09 fuses, -and in particular not to blow at three times the rated current in times less than 6 seconds. The structure of FIGS. 1 and 2 allows a change of the rated current from 5 to 10 amps by changing the thickness of the fuse link, i.e. without effecting any other change. It forms two series breaks both on occurrence of protracted overload currents, and on occurrence of major fault currents, or short-circuit currents. However, depending upon the parameters of the particular circuit the fuse of FIGS. 1 and 2 may fail on major fault currents, or short-circuit currents. This is surprising since in fuses having a small rated current two series breaks formed by a pair of serially related perforations are normally adequate to interrupt major fault currents if the circuit voltage is 250 volts, or less. One of the reasons which explains this `behavior of the structure 0f FIGS. 1 and 2 is the compactness of the casing 1 thereof, and another reason which probably accounts for this behavior is the nature of the metal of which fuse link 4 is made.

One of the basic ideas underlying this invention is to modify the structure of FIGS. 1 and 2 by the addition of two points of reduced cross-sectional area formed by two perforations without significantly changing the thermal conditions to which the link-severing overlay is subjected. This seemingly impossible task can be achieved `by two critical changes of the structure of FIGS. 1 and 2. The width of the points of reduced cross-sectional area must be increased. To be more specific, the diameter of the perforations of FIGS. 1 and 2 in the fuse link must be reduced from 0.124 inch to about `0.1 inch, preferably to 0.109 inch. This reduces the generation of heat to which the link severing overlay is subjected by the two points of reduced cross-sectional area immediately adjacent thereto. The addition of the two perforations each having a diameter of about 0.1 inch results in two heat sources not present in the structure of FIGS. 1 and 2. I have found that the presence of these two additional perforations is permissible from the thermal point of view, i.e. does not change the time-current characteristic of the structure of FIGS. 1 and 2, if the spacing of the center of each of the two additional perforations from the end surfaces of ferrules 2 immediately adjacent thereto is about 0.23 inch. The preferred or optimal spacing is 0.2345 inch. Deviations of this spacing must be less than M22 inch, and should be minimized. At the critical spacing of 0.23 inch the following two conditions are met.

(1) The axially outer added perforations are so close to the ferrules 2 as to constitute the latter heat sinks sufciently effective to absorb at currents up to 3 times the rated current of the fuse all the excess heat generated by the added axially outer perforations. Let H1 be the heat generated in the structure of FIGS. 1 and 2 at the two points of reduced cross-section formed by the two serially related perforations each having a diameter of 0.124 inch and conducted to the link-severing overlay, let H2 -be the heat generated at the two points of reduced crosssectional area `formed by the two axially inner perforations each having a diameter of 0.109 inch (taking the place of the two perforations of FIGS. l and 2) and conducted to the heat-severing overlay, let H3 be the portion of the heat generated at the two axially outer points of reduced cross-section formed by the two axially outer perforations having a diameter of 0.109 inch which is conducted to the link-severing overlay, and let H4 Ibe the portion of the heat generated at the two axially outer points of reduced cross-section formed by the two axially outer perforations having a diameter of 0.109 inch which is conducted to Iferrules 2, thi following relations obtain: H2 H1 (l) These relations must be met for currents up to three times the rated current of the fuse, and result in the required time-current curve, if met.

(2) The two axially outer points of reduced crosssection are spaced suciently far from the end surfaces of the ferr'ules to preclude burnback into the ferrules, even if the interrupting conditions are relatively onerous. From the aspect of preventing burnback into the -ferrules the axially outer points of break should be spaced as far as possible from the end surfaces of the ferrules. The above thermal requirements, i.e. the use of the ferrules as current-curve determining heat sinks, compel to arrange axially outer points of reduced cross-section, or the axially outer points of break, relatively far axially outwardly but, if the above critical dimensions are being kept, the axially outer points of reduced cross-section, or the axially outer points of break, are still spaced sutilciently far from the end surfaces of the ferrules to preclude burnback of the arc into the end surfaces of the ferrules.

Referring now to FIGS. 3-5, the same numerals with a prime added have been applied in FIGS. 3 to 5 as in FIGS. 1-2 to designate like parts. Thus the fuse strueture of FIGS. 3 and 4 include a tubular casing 1' of electric insulating material having the standard dimensions of the casing of FIGS. 1 and 2 stated above. The ferrules 2' of the structure of FIGS. 3 and 4 are of the same nature as the ferrules of the structure of FIGS. 1 and 2 and have the same dimensions as the ferrules of that structure. The pulverulent arc-quenching filler 3 inside casing 1 has a substantially lower thermal conductivity than fine quartz sand, and about the same thermal conductivity as powdered gypsum having a particle size of 18 to 20 U.S. Standard Sieve Number. The arc-quenching iiller 3 is preferably powdered gypsum having a particle size of 18-20 U.S. Standard Sieve Number. If desired chalk might take the place of gypsum, though the latter is preferred. Ribbon fuse link 4 is of a phospher bronze having a small electrical conductivity. Best results have been obtained with phosphor -bronze having a tin content of 9.0 to 11% and a content of phosphorous of 0.003 to 0.35%. This metal is known as ASTM Specilication B 103Alloy D. It has an electrical conductivity of 11% IACS at 68 deg. F., or an electrical resistivity of 94.3 ohms (circ. mil/ft.) at `68 deg. F. This alloy is particularly desirable since it makes it possible to achieve time-lags beyond and above those specified for Style F09 fuses. The ends of fuse link 4' project through ferrules 2 and are conductively connected to the latter by pools 5 of soliditied solder. Link-severing overlay 6 which includes tin, or consists of tin, is arranged in the center of the fuse link and has the same geometry as the overlay of the structure of FIGS. 1 and 2. Fuse link 4 defines four points of reduced cross-sectional area which may or may not be formed by perforations, eg. circular perforations. If formed by circular perforations of equal diameter their diameter rnust be about 0.109 inch. Thus the width of each point of reduced cross-section is 0.150l inch (the width of fuse link 4') minus 0.109 (the diameter of the perforation) which is 0.41. Each of the points of reduced cross-sectional area may be formed by pairs of lateral incisions leaving a neck having a width of 0.041 inch. Such a neck is the equivalent of a pair of parallel-connected necks having a total width of 0.041 inch, formed by a circular perforation having a diameter of 0.109 inch. In other words, a single neck may take the place of two necks in parallel, provided that the neck Width is the same in both instancesi.e. 0.041 inch--and further provided that none of the above critical dimensions undergoes any change. For a comparison of the above referred to geometries, i.e. the single neck geometry, and the parallel neck geometry, reference may be had to FIGS. l and 2 of U.S. Patent 2,592,399 to William S. Edsall et al., April 8, 1952 for Current-limiting Fuse. Rectangular perforations as shown in FIG. 2 of the above patent may be substituted for the circular perforations of FIGS. 3 and 4, provided that the total width of the points of reduced cross-sectional area- 0.041 inch-is not altered, and that all other critical dimensions of the structure `of FIGS. 3 and 4 are retained. The spacing of each of the centers of the axially outer points of reduced crosssectional area from the end surface of the ferrule immediately adjacent thereto is about 0.23 inch, preferably 0.2345 inch, and the latter dimension has been indicated in FIG. 3. FIG. 3 also indicates the preferred size of perforations 4' which is 0.109 inch. The mass of the fuse link 4 and the mass of the overlay 6 are sufficiently large to preclude interruption of the current path formed by fuse link 4 by melting of overlay 6 in times shorter than 6 seconds at currents 3 times the rated current of the fuse.

The above data are the necessary and suflicient conditions for obtaining static Style F09 fuses fully com.- plying with all electrical and thermal requirements irnposed on such fuses. The structure of FIGS. 1 and 2 is only capable of meeting the thermal requirements imposed on a static Style F09 fuse, but not the electrical requirements imposed on such a fuse. Structures involving ribbon fuse links having three serially related points of reduced cross-sectional area and a link-severing overlay of a low fusing metal, such as tin, are not capable of meeting with all thermal `and electrical requirements imposed on static Style F09 fuses. The dimensional data which have been given above are the prerequisites for obtaining otpimal results, provided that all serially related points of reduced cross-sectional area have the same width and are spaced equidistantly, or have a spacing of, or close to, 11/32 inch. To provide points of reduced crosssectional area which have the same width and are spaced equidistantly greatly simplifies manufacturing of static Style F09 fuses.

It is, however, of interest to investigate how, and to what extent, changes in regard to dimensions affect the performance of static Style F09 fuses. Increasing the diameter of the axially outer perforations to valves more than 0.10 inch increases the heat generated at the axially outer points of reduced cross-sectional area, thus upsetting the thermal conditions resuired for achieving the prescribed time-current characteristic. The excess heat generated at axially outer points of reduced cross-sectional area having a width of significantly less than 0.04 inch and/or that are for-med by circular perforations having a diameter of significantly more than 0.10 inch cannot be compensated by increasing the axial heat tiow by reducing the spacing of the axially outer points of reduced cross-sectional area `from the end surfaces of the ferrules to significantly less than the critical value of 0.23 inch. There are two reasons precluding such a change, i.e. the reduction of backburn length of the axially outer breaks resulting from such a change is not tolerable, and the heat absorbing capacity of the ferrules is not sufficient to absorb the excess heat which would result from a significant decrease of the critical Width of the axially points of reduced cross-sectional area.

It is permissible, on the other hand, to reduce the diameter of the axially outer perforations below the critical valve of 0.109 inch, provided that the axially inner perforations retain their critical diameter of 0.109 inch. Such a reduction of the diameter of the axially outer perforations results in a reduction of the i2-r heat or Ioules heat generated at the axially outer points of reduced cross-sectional area, resulting, in turn, in an insufticient heating of the link-severing overlay to achieve the prescribed time-current curve. This deficiency can, however, be compensated -by arranging the axially outer points of reduced cross-sectional area closer to the axially inner points of reduced cross-sectional area, i.e. by increasing the spacing between the axially outer points of reduced cross-sectional area and the end surfaces of the ferrules above the critical value of 0.23 inch. Such increase is desirable since it increases the backburn length of the axially outer points of break,

It is apparent from the foregoing that static Style F09 fuses may be evolved in two different ways, namely providing the fuses with four identical points of reduced cross-sectional area and either by adhering closely to the critical numerical values which have been set forth above, or by slightly increasing the width of the axially outer points of reduced Cross-sectional area above the critical data which have been set forth above, and concomittantly increasing the spacing between the axially outer points of reduced cross-sectional area and the end surfaces of the ferrules above the critical spacing of 0.23 inch.

It will be apparent to those skilled in the art that some changes and modifications may be made in the fuse structure which has been described above without departing from the spirit of the invention as set forth in the appended claims.

I claim as my invention:

1. An electric military standard Style F09 cartridge fuse comprising in combination:

(a) a tubular casing of an electric insulating material having a length of about 11/2 inches and an outer diameter of about 0.4 inch;

(b) a pair of ferrules mounted on said casing and closing the ends thereof, each of said pair of ferrules overlapping the side wall of said casing about 0.4 inch;

(c) a pulverulent arc-quenching filler inside said casing having a substantially lower thermal conductivity than the thermal conductivity of fine quartz sand and about the same thermal conductivity as powdered gypsum having a particle size of 18-20 U.S. Standard Sieve Number;

(d) a ribbon fuse link of phosphor bronze having a small electrical conductivity and having a width of about 0.150 inch arranged inside said casing, submersed in said arc-quenching ller and conductively interconnecting said pair of ferrules, said fuse link having a link-severing overlay including tin substantailly at the center thereof and said fuse link defining four serially related points of reduced crosssectional area each having a width of about 0.04 inch, the spacing of each of the axially outer of said four points of reduced cross-sectional area from the end surface of one of said pair of ferrules immediately adjacent thereto being about 0.23 inch, and the mass of said fuse link and the mass of said link-severing overlay being suiiiciently large to preclude interruption of the current path formed by said fuse link by melting of said link-severing overlay in times shorter than 6 seconds at currents 3 times the rated current of said fuse.

2. An electric cartridge fuse as specified in claim 1 wherein said ribbon fuse link defines four substantially equidistantly spaced circular perforations each having a diameter of about 0.10 inch and less than 0.12. inch, the center of each of the axially outer perforations of said four perforations from the end surface of one of said pair `of ferrules immediately adjacent thereto being about 0.23 inch.

3. An electric fuse as specified in claim 2 wherein said ribon fuse link is of phosphor bronze having an electric conductivity of about 11% IACS at 68 deg. F., a tin content of about 9.0 percent to 11.0 percent, and a content of .phosphorous of about 0.03 percent to 0.35 percent.

4. An electric military standard Style F09 fuse cornprising in combination:

(a) a tubular casing of an electric insulating material having a length of about 11/2 inches and an outer diameter of about 0.4 inch;

(b) a pair of ferrules mounted on said casing and closing the ends thereof, each of said pair of ferrules overlapping the side wall of said casing about 0.4 inch;

(c) a pulverulent arc-quenching filler inside said casing having a substantially lower thermal conductivity than the thermal conductivity of fine quartz sand and about the same thermal conductivity as powdered gypsum having a particle size of 18-20 U.S. Standard Sieve Number; and

(d) a ribbon fuse link of Phosphor bronze having a small electrical conductivity and having a width of about `0.150 inch arranged inside said casing, submersed in said filler, and conductively interconnecting said pair of ferrules, said fuse link having a linksevering overlay including tin substantially at the center thereof and said fuse link defining four serially related points of reduced cross-sectional area including a pair of axially inner points of reduced crosssectional area and a pair of axially outer points of reduced cross-sectional area, each of said pair of axially inner points of reduced cross-sectional area having a width of about 0.04 inch and each of said pair of axially outer points of reduced cross-sectional area having a larger width than each of said pair of axially inner points of reduced cross-sectional area, each of said pair of axially outer points of reduced cross-sectional area having a spacing fromv the end surface of one of said pair of ferrules immediately adjacent thereto of less than 0.23 inch, and the mass of said fuse link and the mass of said link-severing overlay being sufiiciently large to preclude interruption of the current path formed by said fuse link by melting of said link-severing overlay in times shorter than 6 seconds at currents 3 times the rated current of said fuse.

5. An electric fuse as specified in claim 4 wherein said ribbon fuse link defines two axially inner perforations each having a diameter -of about 0.10 inch and less than 0.12 inch, wherein said fuse link further defines two axially outer perforations each having a diameter of less than 0.10 inch, each of said axially outer perforations having a center spaced less than 0.23 inch from the end surface of one of said pair of ferrules immediately adjacent theret0.

6. An electric fuse as specified in claim v5 wherein said ribbon fuse link is of phosphor bronze having an electric conductivity of about 11% IACS at 68 deg. F., a tin content of about 9 percent to 11.0 percent, and a content of phosphorous of about 0.03 percent to 0.35 percent.

References Cited UNITED STATES PATENTS 2,703,352 3/1955 Kozacka 337--163 3,291,942 12/1966 Kozacka 337--163 BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner.

U.S. Cl. X. R. 337-163, 182 

